1. Field of the Invention
The present invention relates to a gas vent system of a mold.
2. Description of Related Art
A molding machine is generally provided with a gas vent unit in order to prevent failures or defects such as gas holes, shrinkage voids or the like in molding or casting. For instance, die castings are made by forcing molten metal under high pressure into a metal mold, so that they can provide die castings with fine metal textures, good die-cast surface smoothnesses, and close dimensional tolerances. In the die castings, the molten material is forced under high pressure into a mold cavity of the mold so that any gases in the cavity should be allowed to escape toward the outside.
The gas vent unit equipped in the mold usually comprises a gas vent passageway defined between mating surfaces of the mold halves and communicated with the outside of the mold from the mold cavity, when the mold halves are closed, and a gas vent valve mounted on one of the halves.
Japanese Patent Examined Publication (kokoku) No. 14,229/1984 discloses a gas vent system of conventional type, in which a gas vent valve is disposed in a position perpendicular to a gas vent passageway and the vale is designed to be closed to shut off the gas vent passageway from the outside only when the passageway is filled with the molten material forced into the mold cavity. In other words, the gas vent valve starts operating to close the gas vent passageway when the molten material forced into the cavity then fills the gas vent passageway extending from the cavity to the gas vent valve.
This is not preferred, however, in terms of sensitivity of the gas vent valve. In the initial stage when the molten material is extended to the gas vent valve, a small amount of gases held in the gas vent passageway is still being discharged toward the outside. Accordingly, it is preferred that the gas vent valve starts operating to close the gas vent passageway from the initial stage when the molten material has reached the gas vent valve.
The gas vent unit of such conventional type as described hereinabove is further designed to allow a whole surface area of its end face of the gas vent valve, which faces the gas vent passageway, to receive the pressure of the molten material. This requires the gas vent passageway to pass through a whole length of a peripheral portion of the facing end face of the gas vent valve, thereby incurring the risk that the molten material is leaked from a portion where the gas vent valve is mated with the mold have to which the valve is mounted.